The upstream sensor should be switching rapidly full range between .1 volts and .9 volts. .1 is lean and .9 is rich. The switching would be from the pulsation from the opening and closing of the exhaust valves. If it is staying between .1 and .4 it is reading a lean condition or it is malfunctioning. Try driving the system rich to see what happens to the readings. You can drive it rich by spraying gas into the intake (very dangerous) or by using an unlit propane torch.
The downstream sensor should not switch nearly as rapidly. That is how the computer tells if the cat is functioning well or not. When the downstream sensor too closely mirrors the upstream sensor, the computer says "tilt" and turns on the check engine light and sets a code. I don't know what the ratio paramaters are.
